1. Field of the Invention
This invention relates to a transfer material conveying apparatus for conveying a transfer material to which toner images are heated and fixed and to an image forming apparatus using a heat-fixing method.
2. Description of Related Art
Image forming apparatuses such as electrophotographic apparatuses are becoming faster, multi-colored, and highly functioned these days, and printers of various types are commercially available. From a viewpoint to faster printers, apparatuses using inline types in which images are formed in aligning plural electrophotographic units for forming images in respective colors and in driving those units at the same time, are researched and developed extensively, and those apparatuses possess great potential to be used widely in business because those apparatuses are able to form multicolor images with high speed.
Particularly, inline printers of conveyance belt drive types are frequently developed in which plural electrophotographic unites, in each of which electrophotographic processes such as charge, exposure, development, and cleaning are done in a united body, are aligned linearly for transferring toner images onto transfer materials from the plural electrophotographic units where the transfer materials, or sheets for recording, are attached onto a conveying belt also serving as a transfer belt, because rendered easily compact and inexpensive from a smaller number of process structure elements.
FIG. 5 shows an apparatus structure example of a conventional inline type apparatus. The conventional apparatus has a layout that process stations 20M, 20C, 20Y, 20K in colors of magenta (M), cyan (C), yellow (Y), black (K) placed in a horizontally extending line at a peripheral face of a transfer material conveyance belt (e.g. an electrophotographic transfer belt or ETB) 1 tensioned with a drive roller 2 and a tension roller 3.
A surface of a photosensitive drum 6 in each process stations 20M to 20K is charged uniformly with an electric charger 7, and latent images are formed on the surface with an exposing optical system 8. These latent images are developed by a developing unit 9 to be visualized as toner images. The toner images in respective colors on the respective photosensitive drums 6 are then transferred to a transfer material on a conveyance belt 1 with a transfer roller 4 at a transfer section facing to the photosensitive drum 6, thereby forming a multicolor image by overlaying the four color toner images on the transfer material. Toners remaining after transfers of toner images on the photosensitive drums 6 are wiped out by a cleaner 10 to render the surfaces of the photosensitive drums 6 clean.
The transfer material is fed from a feeding cassette 14, and the transfer material is introduced by a pickup roller 15 to the conveyance belt 1 and is attracted electrostatically to the conveyance belt 1 by application of bias voltage when passing through a nipping section constituted of an attracting roller 5 and a tension roller 3. The transfer material attracted to the conveyance belt 1 is conveyed in a horizontal direction as shown by an arrow by the conveyance belt 1.
The transfer material to which the toner images in four colors are transferred is separated in aid of the curvature at a rear end in the downstream side of the conveyance belt and is conveyed to a fixing unit 11 to be subject to fixing process. The fixing unit 11 includes a heating roller 121 having a halogen heater 121a, a fixing section 12 having a pressing roller 122 in pressurized contact with the heating roller 121, and a delivery section 13 made of a pair of delivery rollers 131, 132 formed at the outlet of the fixing section 12.
The four-colored toner images are fixed to the transfer material upon application of heat and pressure when the transfer material is nipped and conveyed to the fixing nipping section constituted of the heating roller 121 and the pressing roller 122. The transfer material to which the toner images are fixed at the fixing section 12 is introduced to the exterior of the fixing unit 11 through the delivery section 13, and is stacked finally on a delivery tray 16.
Furthermore, an inline printer of a vertical conveyance type is recently developed in which the electrophotographic units constituting processing stations are aligned vertically for reducing the installation area size. This printer has a figure that the inline printer of the horizontally conveyance type shown in FIG. 5 is rotated for 90 degrees, and as the transfer material attracted onto the conveyance belt is conveyed upward in opposing to the gravity, the respective process stations transfer the toner images in the respective colors to form full color images on the transfer material and to fix the images with heat by means of the fixing unit mounted at a top of the apparatus.
In addition, an image's transparency when an image is to be outputted on an OHP (Over Head Projector) sheet as a transfer material is needed as a specification required by a color printer to form the image.
Because the OHP sheets are made of a synthetic resin film such as PET (polyethylene-terephthalate) having a thickness about 100 microns and have a larger heat capacity than ordinary transfer materials such as plain paper, a heat supply is much more substantial than an ordinary heat supply is required for fixing process. It is necessary to melt surely the toner images on the OHP sheet and to render the surface of the toner images smooth to order to obtain good transparency. When the transferred toner images are fixed on the OHP sheet, the heat amount applied per unit time is increased by raising the fixing temperature or reducing the conveyance speed for fixing process.
In a case of inline printers of the horizontal conveyance type in which the electrophotographic units are aligned horizontally, the fixing unit is disposed beside the conveyance belt, and the positional relation between the heating roller and the pressing roller in the fixing unit is of a layout that the heating roller exists over above the pressing roller. As shown in FIG. 6, when the OHP sheet S is passed to the fixing nipping section, the OHP sheet S softened due to heat is delivered to the exterior of the apparatus by the delivery roller pair 131, 132 in forming a route extending downward as shown with solid line Ts where the OHP sheet S takes the delivery direction directing downward, or namely closely to the pressing roller 122 by the self-weight.
On the other hand, in a case of inline printers of the vertical conveyance type in which the electrophotographic units are aligned vertically, operation is done as shown in FIG. 7. With this structure, the fixing unit 11 is located over the conveyance belt, and the positional relation inside the fixing unit 11 is of a layout that the delivery section 13 is located right above the fixing unit 12, thereby rendering the OHP sheet S substantially vertical at the fixing nipping section. As different from the structure of the horizontal conveyance type, the OHP sheet S does not have the effect directing the delivery direction closely to the pressing roller 122 by the self-weight and does not take a stable route while taking a route T0 in FIG. 7 as well as taking routes T1, T2 curving toward the sides of the heating roller 121 and the pressing roller 122, respectively.
Accordingly, with the apparatus structure as shown in FIG. 8, it is necessary to design the length between the fixing nipping section and the delivery roller pair 131, 132 to be shorter or to provide some guide in order to render the sheet conveyance route always. e.g., the route T2.
When the OHP sheet S passes through the delivery roller pair 131, 132 as shown in FIG. 8, however, the OHP sheet P is subject to very high temperature, and therefore, when the delivery roller 131 divisionally arranged in a longitudinal direction, or namely the perpendicular direction to the sheet conveyance direction, comes in contact with the side on which the toner images are formed, the toner image surface at a high temperature is partly pushed down by the delivery roller 131, thereby raising problems to generate unevenness in smoothness, to produce stripe-shaped unevenness (or roller mark) when the OHP sheet S is subject to projection, and to render the projection image quality of the color images very inferior. This is a problem more easily occurring as the fixing nipping section is located more adjacently to the delivery roller pair 131, 132. Such a problem occurs frequently even in the structure as shown in FIG. 6, and occurs very frequently in the structure shown in FIG. 7 where the delivery roller pair 131, 132 is placed closely to the fixing nipping section.
To prevent this problem from occurring, a possible solution is to make the delivery roller pair wider, or namely having a width comparable to the sheet width, in the longitudinal direction as a pair of delivery rollers 133, 134 as shown in FIG. 9, but such a situation raises a new problem that the OHP sheet S is abruptly cooled with the delivery roller pair 133, 134 to generate curling.
It is an object of the invention to prevent scars such as roller marks or the like at images on the sheet from occurring as well as to prevent curls of the sheets from occurring.